TRAVERSE CITY -- An environmental group that coordinates beach cleanup programs Thursday joined the U.S. Coast Guard in the search for the source of household and street garbage washing up on Lake Michigan beaches.

The Chicago-based Alliance for the Great Lakes called for volunteers to scour beaches along Michigan's western coastline. They were asked to collect debris and record any labels or other information that would help determine where it came from.

Garbage has shown up in recent weeks in scattered locations as far south as Saugatuck and as far north as Sleeping Bear Dunes National Lakeshore in Michigan's northwestern Lower Peninsula -- a roughly 200-mile zone.

The biggest amount was in the city of Manistee, where hundreds of pounds of junk washed up the night of July 13. A public beach was closed temporarily while the mess was hauled away.

Some items had printed addresses from the Milwaukee area. Ironically, a "No Dumping Allowed" sign from the Milwaukee County Park Commission was among the trash found in Manistee.

Even so, authorities with the federal government and both states said it's too early to point the finger at Wisconsin or anywhere else. Nor is it certain all the garbage came from the same place, said Lt. David French, spokesman for the Coast Guard's 9th District headquarters in Cleveland.

"We certainly don't want to rule anything out or in at this point," French said. "Our investigators are tracking down a lot of leads."

In Milwaukee, Department of Public Works spokeswoman Cecilia Gilbert said a local investigation hit a dead end.

The city sanitation department collects residential household waste, then hauls it to landfills that are not near the lakefront, she said. It does not license barges to carry garbage on the Great Lakes.

"It's really a very odd situation," Gilbert said. "I can't wait to see how this mystery is going to be solved myself."

Searchers also haven't found many tampons, condoms or other material that would point to sewage overflows as the likely culprit, said Jamie Cross, the Alliance for the Great Lakes' Adopt-a-Beach coordinator in Michigan.

Beach litter is nothing new around the Great Lakes. A number of organizations sponsor annual cleanups that collect huge amounts of trash, particularly cans, bottles, cigarette butts and food packaging. The Petoskey-based Tip of the Mitt Watershed Council is planning an Aug. 16 beach sweep for Cheboygan, Emmet, Charlevoix and Antrim Counties.

But it's unusual for large volumes of refuse to drift ashore from faraway locations, French said.

Medical waste washed up on beaches in Lake Erie and Lake Michigan nearly two decades ago, making national headlines.

A few empty syringes, tongue depressors and similar items were among the recently discovered trash. But they weren't considered medical waste because they didn't contain bodily fluids or tissues, said Robert McCann, spokesman for the Michigan Department of Environmental Quality.